Banfield, the only unbeaten team, remained level with Newell's Old Boys, who beat River Plate 2-1 on Saturday, and three points above Estudiantes, who crushed title holders Velez Sarsfield 3-0 in La Plata.
Fernandez, who is in the Uruguay squad for the World Cup playoff against Costa Rica on November 14 and 18, ran on to a through ball from Walter Erviti to hit a rising drive from just inside the box past keeper Pablo Migliore and in off the underside of the bar in the 72nd minute.
Estudiantes, the highest scoring team with 24 goals in 13 matches, went ahead when central defender Leandro Desabato dived to head home Leandro Benitez's free kick just past the half hour.
Striker Mauro Boselli increased the South American champions' lead early in the second half and Benitez rounded off a brilliant 16-pass move for the third eight minutes from time.
In Rosario on Saturday, lanky Uruguayan striker Joaquin Boghossian scored both goals against ailing giants River.
An editorial in the sports daily Ole on Sunday written from River's point of view carried the headline: "He's two metres tall, but we still lost sight of him".
Boghossian, 1.95 metres tall, lobbed Newell's first and headed home the game winner after Diego Buonanotte had equalised for River, whose defeat leaves them all but out of South American club competition in 2010 for the first time in 23 years.
River, with only 10 points from their 13 matches, are 19 points behind the leaders with only three teams below them. They are not so worried about relegation because it is decided on average points over three seasons.
Their arch-rivals Boca Juniors were held 0-0 at home by fourth-placed Colon in their second match without injured playmaker Juan Roman Riquelme and remain in mid-table 10 points off the pace.
